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The Ideal Op Amp Calculation

One of the most successful electronic devices of all time, the operational amplifier, also known as the op amp, is a model of simplicity. It provides gain for analog signals through its two inputs ̵1; one inverting, one non-inverting. Calculations for an op circuit assume an ideal behavior for the device. Though no real-world op is perfectly ideal, they are usually close enough for the calculations to work for many practical situations.

Ideal Op Amp Features
- An op is as much a concept as it is a device; as a concept, it has several notable ideal characteristics. For example, its inputs draw no current, so they exhibit infinite impedance. Its output has zero impedance, so it delivers any amount of current to other circuits. With no feedback resistor connecting its inverting input to its output, an op has infinite gain; you can use a feedback resistor to produce any finite amount of amplification. Engineers call the no-resistor condition ̶0;open loop,̶1; so an op has infinite open-loop gain. An ideal op also has infinite bandwidth, so it handles signals from zero to infinite Hz.
Inverting Amplifier
- One of the simplest and most common op circuits is the inverting amplifier. One resistor connects from a signal source to the inverting input, and another connects the output to the inverting input. The non-inverting input has a connection to an electrical ground. The output voltage is the inverse of the input, so a positive input signal becomes negative at the output and vice-versa. You calculate the circuit̵7;s gain by dividing the feedback resistor value in ohms by the input resistor value. For example, an ideal op with a 1,000,000 ohm feedback resistor and 1 ohm input resistor has a gain of 1,000,000. A real-world op may not have an open-loop gain of 1,000,000, however, so this isn̵7;t a practical circuit.
Non-Inverting Amplifier
- As with the inverting amplifier, a non-inverting op circuit has two resistors, though the connections are different. One resistor connects the output and inverting input, and the second resistor connects the inverting input to ground. The input signal drives the non-inverting input directly. To calculate the gain factor for this circuit, divide the feedback resistor̵7;s value by the inverting input resistor value, then add 1. If the feedback and input resistors are both 10,000 ohms, then (10,000 / 10,000) + 1 = a gain of two.
Summing Amplifier
- In addition to single-input amplifiers, an op can add the voltages from multiple inputs together. An amplifier may have several input resistors all connected together on one side, and that side feeds the op amp̵7;s inverting input. The gain for any given input is the same calculation as for a single input: the feedback resistor value divided by the input resistor value. The output is the sum of the inputs multiplied by -1. For example, an op has two input resistors, one 10 ohms and one 20 ohms. The feedback resistor is 20 ohms. A one-volt signal drives both inputs. The first input has a gain factor of 20/10 = 2, and the second has a gain of 20/20 = 1. The output is then (2 + 1) * -1 or -3 volts.

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